Liquid crystal display devices have been used for many years and their uses have been concentrated in small appliance applications such as in electronic watches and calculators. LCD's have also been used in applications for instrumental numerical displays and graphical displays. More recently, the development of color LCD's and large screen LCD's have broadened their applications to televisions and notebook computers.
Major benefits for LCD's are their inherent properties of small thickness, lightweight, low driving voltage required and low power consumption. As a consequence, more recent applications of color LCD's can be found in small screen television sets, notebook computer display panels and video camera view finders as replacements for conventional CRT's. A liquid crystal display device can be either a color unit or a black and white unit. The device can also be constructed as a reflecting type or as a transmittive type, depending on the light source used.
Since liquid crystal molecules respond to an externally applied electrical voltage, liquid crystals can be used as an optical switch or as a light valve. A typical liquid crystal display cell arrangement is shown in FIGS. 1A and 1B.
Referring initially to FIG. 1A, wherein a liquid crystal display device 10 is shown. Liquid crystal display cell 10 is a single pixel which is constructed by two parallel glass plates, i.e., an upper plate 12 and a lower plate 14. Both the upper plate 12 and the lower plate 14 have a polarizing film 36 and 32 coated on its outer surface. The cavity 18 formed between the two plates 12 and 14 is filled with a liquid crystal material 20. One of the most commonly used liquid crystal material is of the twisted nematic (TN) type wherein the twist refers to the tendency of the polymer liquid crystal to form chains that rotate from one side 22 of the gap between the plates to the other side 24 of the gap. The degree of rotation can be controlled during the fabrication process.
As shown in FIG. 1A, light beam 28 passes through the polarizer 36 and then through the liquid crystal display cell 10 having its polarization direction rotated by following the physical rotation of the liquid crystal molecules 26. As shown in the simplified illustration in FIG. 1A, the polarizer 32 on the exit side 24 of the liquid crystal cell 10 is positioned such that it allows a rotated light beam 30 to pass through the polarizer 32. When viewed from the side of the polarizer 32, the pixel or the liquid crystal cell 10 thus appears clear, i.e., in a transmitting mode.
A transparent electrical conductor (not shown) such as indium-tin-oxide (ITO) is normally deposited on the inner surfaces of the glass plates 12 and 14. The transparent electrical conductor layer is patterned into a series of mutually perpendicular lines (not shown). When a voltage is applied across the cell cavity 18 by addressing the appropriate line on each side of the cell, the liquid crystal molecules 26 reorient themselves to follow the applied electric field. The liquid crystal materials 26 are thus untwisted as shown in FIG. 1B. The passage of the untwisted light beam 34 is blocked by the exit polarizer 32 as long as the voltage is present. When the voltage is turned off(as shown in FIG. 1A), the liquid crystal molecules 26 returns to their original state and the cell or the pixel becomes clear again. As previously stated, typical voltages and currents required to activate the liquid crystal molecules are relatively low making it an ideal candidate for incorporation in battery-operated equipment where a low power consumption is essential. A typical twisted nematic (TN) liquid crystal cell used for small displays have a twist angle of 90.degree.. More recently developed supertwisted nematic (STN) material forms a twist angle up to 270.degree. and thus allow higher contrast and faster response time so that many pixel elements can be multiplexed in a single display.
While twisted nematic type liquid crystal display devices enjoy the traditional benefits of liquid crystal display cells such as lightweight and low power consumption, they also exhibit disadvantages such as the limitation of a narrow viewing angle on a flat panel device, and the unsymmetrical viewing angles at various viewing positions toward the panel. While others have proposed various techniques for correcting the narrow viewing angle drawback of the liquid crystal display device, none of the techniques proves to be satisfactory. For instance, one method is the utilization of a two-domain twisted nematic mode liquid crystal device for correcting the viewing angles of LCD devices. Even though the technique improves the viewing angle somewhat, the technique requires complex manufacturing process to be used. For instance, two orientation steps must be performed on both the upper and the lower glass substrate such that the fabrication cost and time are significantly increased. Furthermore, the focusing error is increased between the domains on the upper and the lower substrate which increases light leakage and sacrifices the quality of the device produced.
It is therefore an object of the present invention to provide a method for fabricating a liquid crystal display cell that does not have the drawbacks and shortcomings of the conventional fabrication methods.
It is another object of the present invention to provide a method for fabricating a liquid crystal display cell that has improved viewing angle by the addition of only a limited number of processing steps.
It is a further object of the present invention to provide a method for fabricating a liquid crystal display cell by aligning liquid crystal molecules with a predetermined slope formed on a base substrate.
It is still another object of the present invention to provide a method for fabricating a liquid crystal display cell by incorporating steps for forming a multiplicity of elongated steps on the surface of a lower substrate first and then coating the steps with a layer of insulating material such that sloped sidewalls are formed on the steps.
It is another further object of the present invention to provide a method for fabricating a liquid crystal display cell that has improved viewing angle by first depositing a photosensitive layer on a lower substrate, then patterning the layer into a multiplicity of elongated steps and depositing an insulating material layer overlying the steps thus forming sloped sidewalls on the steps.
It is yet another object of the present invention to provide a method for fabricating a liquid crystal display cell which has improved viewing angle by aligning liquid crystal molecules at an angle between about 10.degree. and about 60.degree. when measured from a plane perpendicular to the planar surface of the base substrate.